It is well known that if an electric field is generated between two oppositely charged electrodes, any charged particles which are located between the two electrodes will be attracted to the corresponding electrode having a charge opposite that of the particle. This phenomenon is known as electrophoresis. If the particles do not have a net charge, and are polarizable, the electric field will induce a polarization in the particle; for example in a metal particle this results in a shift of electrons to the side of the particle facing the positive electrode. Since the particle is still neutral, although polarized, no movement occurs in a uniform field. The magnitude of the resultant polarization is related to the effective dielectric constant of the material and of the support medium. Materials which have high dielectric constants exhibit large polarizations in the presence of an electric field. Materials which have a low dielectric constant, on the other hand, develop much lower polarization in the presence of an electric field.
If the particles are placed between two oppositely charged electrodes which produce a non-uniform field, such as that produced when one electrode is a point or line and the other electrode is a plane, the polarized particles will experience a net force tending to move the particle into the region of higher electric field strength, assuming the dielectric constant of the particle is greater than that of the medium in which it is located. It may be noted that the direction of the force is not dependent upon the sign of the electric field but is always in the direction of the field gradient. This motion of matter caused by polarization effects in a non-uniform electric field is known as "dielectrophoresis". The principle of dielectrophoresis has been utilized in various types of apparatus to remove polarizable particles from fluids, by passing the fluid containing the polarizable particles to be removed between two dissimilar electrodes, between which is generated a non-uniform electric field. The most popular electrode configuration for such particle separation apparatus appears to be the concentric cylinder configuration or some variant of it, such as a single wire in the center of a surrounding cylindrical electrode. Other electrode configurations have been used, however. All of these various prior art apparatuses appear to have in common the feature of passing the fluid containing the polarizable particles to be separated between two electrodes which have a configuration other than parallel planes, so that a non-uniform electric field is formed between the electrodes when a current is applied to them.